1. Field of the Invention
The invention relates to an image forming apparatus which can be used in printing devices, such as a copying machine, printer, plotter, or facsimile.
2. Description of Related Art
Conventionally, a type of image forming apparatus is disclosed in European Patent Publication No. 587/366. The image forming apparatus disclosed in this publication is one designed to apply a drive signal according to an image signal to an aperture electrode unit having a plurality of small holes (which will be hereinafter referred to as apertures), thereby controlling the passage of toner particles through the apertures and forming an image on an image recording medium such as a sheet of printing paper by the toner particles passed through the apertures.
Such an image forming apparatus 100 is shown in FIG. 3. As shown in FIG. 3, a toner carrying roller 114 and a sheet of paper 120 are opposed to each other with an aperture electrode unit 101 sandwiched therebetween. A back electrode roller 122 is provided above, i.e., on the back or opposite side of the paper 120.
An enlarged perspective view of the aperture electrode unit 101 of the image forming apparatus 100 is shown in FIG. 4. The aperture electrode unit 101 comprises a polyimide insulating sheet 102 having a thickness of 25 .mu.m, a plurality of apertures 106 each having a diameter of 100 .mu.m, formed through the insulating sheet 102 and arranged in a line in the longitudinal direction of the insulating sheet 102, and a plurality of control electrodes 104 respectively formed around the plural apertures 106, each control electrode 104 being formed from a copper foil and having a thickness of 1 .mu.m and a width of 20 .mu.m. As shown in FIG. 3, the aperture electrode unit 101 is placed in such a manner that the control electrodes 104 face the paper 120. In the image forming apparatus 100, a control voltage according to an image signal is applied from a control voltage applying circuit 108 to the control electrodes 104 to thereby control passage of a toner 116, carried by the toner carrying roller 114, through the apertures 106. Further, a voltage of +1 kV is always applied from a DC power supply 124 to the back electrode roller 122 to strongly attract the toner 116 passed through the apertures 106 toward the paper 120, thus forming an image on the paper 120.
In the conventional image forming apparatus as mentioned above, however, the voltage of +1 kV is always kept applied to the back electrode roller 122 in a powered condition of the apparatus. In other words, the voltage of +1 kV is constantly applied to the back electrode roller 122 even in a non-printing condition of the apparatus so long as power is applied. Accordingly, in the event that a residual toner deposited in the vicinity of the apertures erroneously passes through the apertures in the non-printing condition, the residual toner having passed through the apertures is attracted toward the back electrode roller by an attracting electric field formed between the control electrodes and the back electrode roller. Such attraction of the uncontrolled toner toward a non-printing portion causes fog on the paper when the paper is present at a position above the apertures, resulting in stain of the paper with the toner, or alternatively causes a stain on the back electrode roller with the toner when the paper is absent at the position above the apertures, which is subsequently placed on the back of the next sheet of paper to be printed. As a result, a deterioration in the quality of an output image is caused in both cases.